Improve APNs http2 connection health check

[yokotaso]

From Best Practices for Managing Connections,

You can check the health of your connection using an HTTP/2 PING frame.

And I happened when gorush try to send push to APNs, But cached http/2
connection is not available.Therefore I thik it is good that gorush
check APN/2 connection availability by HTTP/2 PING frame.

Here is a test code in local. With environment variable GODEBUG=http2debug=2, you can see a ping/2 log.

func TestHTTP2Ping(t *testing.T) {
	PushConf.Ios.Enabled = true
	PushConf.Ios.KeyPath = "../certificate/authkey-valid.p8"
	PushConf.Ios.TeamID = "example.team"
	PushConf.Ios.KeyID = "example.key"
	PushConf.Core.HTTPProxy = ""
	err := InitAPNSClient()
	assert.Nil(t, err)
	assert.Equal(t, apns2.HostDevelopment, ApnsClient.Host)

	for i := 1; i <= 10; i++ {
		if resp, err := ApnsClient.HTTPClient.Head(apns2.HostDevelopment); err != nil {
			t.Fatal(err)
		} else {
			assert.Equal(t, 405, resp.StatusCode)
			time.Sleep(time.Second * 10)
		}
	}
}

Ping log looks like below:

2021/05/04 17:17:20 http2: Framer 0xc0015d49a0: wrote PING len=8 ping="F\x9a\x11\x9b\xb7\x17\v*"
2021/05/04 17:17:20 http2: Framer 0xc0015d49a0: read PING flags=ACK len=8 ping="F\x9a\x11\x9b\xb7\x17\v*"
2021/05/04 17:17:20 http2: Transport received PING flags=ACK len=8 ping="F\x9a\x11\x9b\xb7\x17\v*"
2021/05/04 17:17:21 http2: Framer 0xc0015d49a0: wrote PING len=8 ping="\x05\xf1\xb8PP\xcdp\x99"
2021/05/04 17:17:21 http2: Framer 0xc0015d49a0: read PING flags=ACK len=8 ping="\x05\xf1\xb8PP\xcdp\x99"
2021/05/04 17:17:21 http2: Transport received PING flags=ACK len=8 ping="\x05\xf1\xb8PP\xcdp\x99"

[appleboy]

@yokotaso Thanks.